1. Field of the Invention
The present invention relates in general to an apparatus for controlling an automotive vehicle having an engine and an automatic transmission, and more particularly to techniques for temporarily increasing an output of the engine upon a shift-down action of the automatic transmission during deceleration of the vehicle.
2. Discussion of the Related Art
An automotive vehicle is known wherein the output of an engine is controllable independently of an operation of an accelerator pedal, and an automatic transmission is automatically shifted to a selected one of a plurality of operating positions having respective different speed ratios, depending upon a running condition of the vehicle. For this type of automotive vehicle, there has been proposed a control apparatus adapted to increase the engine output upon a shift-down action of the automatic transmission during running of the vehicle in a decelerating state with the accelerator pedal placed in its non-operated position, so that an engine braking force produced as a result of the shift-down action is reduced so as to mitigate a shifting shock of the automatic transmission and improve the driving comfort of the vehicle. An example of such a vehicle control apparatus is disclosed in JP-A-63-284039.
The known vehicle control apparatus of the type described above uses an idling speed control valve (ISC valve) to increase the engine output upon a shift-down action of the automatic transmission, by increasing the idling speed of the engine while the accelerator pedal is placed in the non-operated position. The ISC valve is activated in response to an engine output increasing command which is generated when the apparatus determines that the automatic transmission should be shifted down during the vehicle deceleration. There exists a time delay between the moment of generation of the engine output increasing command and the moment of actual increase of the engine output by activation of the ISC valve. However, the amount of increase of the engine output by activating the ISC valve is not determined with the time delay taken into account. Consequently, the known apparatus does not necessarily permits a required amount of increase of the engine output upon engagement and/or disengagement of appropriate frictional coupling devices which are used to achieve the shift-down action of the automatic transmission. Therefore, the known apparatus is not capable of sufficiently reducing the shifting shock of the automatic transmission.
In the known vehicle control apparatus, the amount of increase of the engine output, namely, the amount of operation of the ISC valve during a shift-down action of the automatic transmission is determined so that an actual value of an input shaft speed Nx of the automatic transmission after the shift-down action coincides with an target or desired value which is estimated on the basis of the running speed of the vehicle (output shaft speed of the automatic transmission) during the deceleration, and the speed ratio of the operating position of the automatic transmission which is established by the shift-down action. The running speed of the vehicle at which the frictional coupling devices are operated to achieve the shift-down action of the automatic transmission varies depending upon the deceleration value of the vehicle, which is influenced by the gradient of the road surface and the operating state of a vehicle braking system (depending upon whether the braking system is operated or not, and depending upon the braking force acting on the vehicle), even if the amount of operation of the ISC valve is determined at the same vehicle running speed. Therefore, the known apparatus does not assure a suitable amount of increase of the engine output at the time of operation of the frictional coupling devices to achieve the shift-down action of the automatic transmission.